The structure of a conventional ordinary excavator is shown in FIG. 14. A working front device 100 is made up of two members, i.e., a boom 101 and an arm 102. A bucket 103 for use in excavation work is provided at a tip end of the working front device 100. Such an excavator is called the 2-articulation type because the bucket 103 serving as a main member to carry out the work is positioned by two rotatable structural elements, i.e., the boom 101 and the arm 102.
Meanwhile, the so-called two-piece boom type excavator has been employed recently. One example of the two-piece boom type excavator is shown in FIG. 15. The two-piece boom type excavator is modified from the ordinary excavator, shown in FIG. 14, in that a boom 101 of a working front device 100A is divided into two parts, i.e., a first boom 104 and a second boom 105. Such a two-piece boom type excavator is called here a 3-articulation type excavator based on the number of articulations which take part in positioning a bucket 103.
The 3-articulation type excavator has an advantage of enabling the work to be easily carried out near an undercarriage of the excavator, which has been difficult for the 2-articulation type excavator. More specifically, although the 2-articulation type excavator can also be operated to take a posture shown in FIG. 14 for bringing the bucket 103 to a position near the undercarriage, the excavation work cannot be performed with the arm 102 positioned so horizontally as illustrated. On the other hand, in the 3-articulation type excavator, the bucket 103 can be brought to a position near the undercarriage with the arm 102 held substantially vertical as shown in FIG. 15, allowing the excavation work to be carried out near the undercarriage. Further, the excavation work in a position away from the undercarriage can be performed up to a farther position than reachable with the 2-articulation type excavator by extending the first boom 104 and the second boom 105 to lie almost straight.
Another advantage of the 3-articulation type excavator is in enabling the excavator to turn with a reduced radius of turn. When the direction of the working front device 100A is changed by turning an upper turning structure 106 for loading dug earth and sand on a dump car or the like, it is difficult for the 2-articulation type excavator to reduce the radius necessary for the turn because the boom 101 has a large overall length. In the 3-articulation type excavator, the radius necessary for the turn can be reduced by raising the first boom 104 to take a substantially vertical posture and making the second boom 105 extend substantially horizontally. This means that the 3-articulation type excavator is more advantageous in carrying out the work in a narrow-space site.
Next, the conventional operating method will be explained. FIG. 16 shows one example of control levers for use in an ordinary 2-articulation type excavator. In normal excavation work, four kinds of operations effected by the boom, the arm, the bucket and the turn are carried out frequently in a combined manner. These four kinds of operations are allocated to two control levers 107, 108 such that each control lever instructs the two kinds of operations. The excavation work is performed by an operator manipulating the respective levers with the left and right hands. As another control lever, there is a (not-shown) travel lever (usually associated with a pedal as well). The travel lever is used independently of the other levers 107, 108 in many cases; hence it is not here taken into consideration.
FIG. 17 shows one example of control levers for use in a 3-articulation type excavator. As mentioned above, the 3-articulation type excavator can be operated to carry out the work over a wide range from a further position to a position nearer to its undercarriage. To realize this, however, the second boom 105 must also be operated in addition to the first boom 104 corresponding to the boom 101 of the 2-articulation type excavator. Since the four kinds of operations are already allocated to the two control levers 107, 108, a seesaw type pedal 109 is newly provided to operate the second boom 105. See FIG. 4 of JP, A, 62-33937, for example.
Further, JP, A, 7-180173 proposes a control system for a 3-articulation type excavator. According to the proposed control system, two control levers are designed to instruct moving speeds of a bucket tip in the X- and Y-directions, respectively, and a predetermined calculation process is executed based on a resultant speed vector signal of those moving speeds. As a result, in horizontal drawing work, movement of the bucket tip can be controlled continuously over a wide range and the bucket can be moved along a desired path with high accuracy.